Color transparencies produced by electrophotographic techniques



July 29, 1969 J. GAYNOR COLOR TRANSPARENCIES PRODUCED BY ELECTROPHOTOGHAPHIC TECHNIQUES Filed July 1s. 1965 Jo 6a nor;

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United States Patent O COLOR TRANSPARENCIES PRODUCED BY ELECTROPHOTO-GRAPHIC TECHNIQUES Joseph Gaynor, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed July 16, 1965, Ser. No. 472,580 Int. Cl. G03g 13/22 U.S. Cl. 96-1.2 15 Claims ABSTRACT OF THE DISCLOSURE A color recording medium and method are described. The recording medium, in one embodiment, includes a transparent, thermally stable substrate, such as a clear glass slide, on one surface of which is an evaporated film of tin oxide. Over this lm is a transparent layer comprising a filter screen plate made up of a large number of small red, green and blue light filter elements over which is formed a transparent layer of a photosensitive material comprising the recording layer and which 4has `substantially panchromatic response to light in a visible spectrum. The recording is made by projecting on the uncoated surface of the transparent substrate the image or pattern to be recorded. This is transmitted therethrough and through the transparent conductive film to the screen plate `whereupon the color image or light pattern is broken up into primary colors by the lter element transmitted to the photosensitive recording layer. The intensity of light causes ychanges in the photosensitive recording layer which permit an electrostatic charge pattern to be formed on the outer surface of the recording layer which is in point-by-point correspondence to the image with respect to both color and intensity. An opaque, electroscopic powder dusted over the surface of the recording layer is attracted to the charge pattern thereon and adheres to the surface in amounts which are directly proportional to the electrostatic charge level at any particular point thereof.

This invention relates to the production of photographic images in color and more particularly to the production of such images by a xerographic powder development technique.

In conventional xerography, a supporting member which is usually either planar or cylindrical in configuration is provided with a surface coating or layer of a material such as selenium, for example, which is a photoconductor. The photoconductive surface is uniformly electrostatically charged in the dark by means of a corona discharge, for example, and an image pattern comprising light and dark areas is projected on the charged surface. Those zones of the surface which are illuminated become electrically conductive and the charges associated with those zones leak H. Those zones which are not illuminated, or are only weakly illuminated, substantially retain their original charge. An electroscopic powder which is attracted by the retained charge is dusted over the surface and clings by electrostatic attraction to those a`r'eas which were not illuminated. The excess powder is removed leaving an image pattern of powder on the surface. If a sheet of paper or of a transparent plastic having the proper dielectric properties is placed in contact with the powder pattern and an electrostatic charge appliedto the sheet, the powder may be transferred from the photoconductive surface to the surface of the sheet. Usually, the powder contains a resinous binder which upon heat- 3,458,309 Patented July 29, 1969 'ice ing, adheres to the sheet. If a coloring agent is incorporated in the powder or if the powder is inherently, colored, the image pattern of light originally projected on the photoconducive surface will be reproduced upon the surface of the sheet in a form which is readily discernible and, if transparent plastic is used, is projectible. Obviously, the image reproduced on the sheet will be the mirror image of the image pattern of light originally projected on the photoconductive surface, but image reversal by optical means is readily attainable. As previously suggested, the electroscopic powder used may be of any desired color, however, because of the nature of the process, a multicolored image having a true color relationship to the projected light image may not be produced as the result of a single projection of the original multicolor light image. Another inherent difficulty with the previously described process is the lack of gray scale in the recorded image, regardless of the color employed in the powder. This is manifested by the lack of continuous tone in the reproduced image and also by the so-called edgeeffect `which is present when the projected image contains relatively large areas which are either only weakly illuminated, or are black. In the reproduced image, only the edge portions of the area which was originally solid black reproduce as black, while the centra-l portion washes out to a gray.

It would be advantageous to be able to record color images utilizing the photoconductive effect by means of a single exposure which would be reproducible and provide true color rendition in both hue and intensity in addition to black and white as well as provide continuous tones without the use of mechanical devices.

It is, therefore, a principal object of this invention to provide a recording medium whereby color images may be faithfully reproduced and projected for viewing.

A further object of this invention is a method for the reproduction of such color images.

Other objects and advantages of the invention will become apparent to those skilled in the art from the following description of the preferred embodiments of the invention taken in conjunction with the accompanying drawing in which:

FIGURE l is a schematic illustration in four parts showing one embodiment of the invention, the four parts being arranged sequentially from left to right;

FIGURE 2 is a schematic view of one form of a screen plate color filter Viewed in the direction of light transmission; and

FIGURE 3 is a schematic illustration of another embodiment of the invention similar to the two left-hand parts of FIGURE 1.

Briefly stated and in accordance with one embodiment of the invention, one surface of a transparent thermally stable substrate, such as a clear glass slide for example, is provided with a transparent electrically conductive coating or layer such as an evaporated film of tin oxide, for example. Over this conductive film is deposited a thin transparent layer comprising a filter screen plate comprised of large numbers of small red, green, and blue light filter elements over which is formed a transparent layer of a photosensitive material comprising the recording layer which has a susbtantially panchromatic response to light in the visible spectrum. A polychromatic image or light pattern is imposed, by projection, for example, upon the uncoated surface of the transparent substrate and is transmitted therethrough and through the transparent conductive film to the screen plate whereupon the color image or light pattern is broken up into the primary colors by the filter elements and transmitted to the photosensitive panchromatic recording layer. The light produces changes in the photosensitive recording layer which are of `an electrical nature permitting an electrostatic charge pattern to be formed on the outer surface of the recording layer which has a point-to-point correspondence to the image or pattern form with respect to both color and intensity. An opaque electroscopic powder dusted over the surface of the recording layer is attracted to the charge pattern thereon and adheres to the surface in amounts which are directly proportional to the electrostatic charge level at any particular portion thereof. It should be noted with respect to this particular embodiment that the electrostatic pattern produced on the recording surface in response to the light image or pattern is such that the highest charge levels are present in those areas of the recording layer which have received the least illumination and the lowest charge levels in those areas which have received the greatest illumination. The excess electroscopic powder, i.e., all .powder granules not electrostatically held on the recording surface, are removed and, in the case where the electroscopic powder contains a thermoplastic adhesive, heat is applied to adhesively secure the opaque powder granules in place. When white light is projected through the transparent substrate, conductive layer, screen plate, and recording layer, the modulated opaque pattern formed by the electroscopic powder particles permits light to pass only in increments corresponding to the form, color, and intensity of those increments in the originally projected color image or pattern of light whereby an image is directly projectible from the transparency thus formed which has a point-to-point correspondence with the original light image not only with respect to shape or form, but also black and white, color and intensity, or gray scale.

More particularly, and with reference to the accompanying drawings, FIGURE 1 illustrates one form of the invention in which a recording medium is formed from a transparent substrate 11 which has a lter screen plate 12 in contact with one of its major surfaces. Screen plate 12 is composed of red, green, and blue light transmission filter elements identified by the letters R, G, and B, respectively. A transparent electrically conductive flm 13 overlies screen plate 12 and a photosensitive layer which comprises a transparent substantially panchromatic photoconductive layer 14 overlies film 13. It will be noted that the transparent substrate 11 may be made from any suitable material such as glass or any of the transparent synthetic organic polymers such as those commonly employed in conventional photographic lm as a base for the photographic emulsion. Such polymeric materials as the polyethylene terephthalates marketed under the names Cronar and Mylar by E. I. du Pont de Nemours and Company, Inc. of Wilmington, Del., are examples of polymers which are satisfactory for this purpose. Furthermore, while the recorded images may be in the form of individual transparencies or slides for projection in a conventional projector, it will be realized that when the flexible plastic polymers are used as the substrate a plurality of individual image patterns may be recorded sequentially on a plastic tape which may be rolled upon a spool for storage and handling.

Filter screen plates suitable for use with this invention may be made in a variety of forms and by several equivalent methods which are discussed in greater detail in U.S. patent application Ser. No. 472,581, now U.S. Patent Number 3,413,117, filed concurrently herewith in the name of Joseph Gaynor, entitled Color Electrophotography and assigned to the assignee of the present invention.

The transparent electrically conductive film 13 may be formed by several well-known methods and may be composed of a wide variety of substances. For example, the vapor deposition of tin transparent electrically conductive 4 layers of metals such as chromium, for example, or compounds such as tin oxide, for example, is well known in the prior art.

The transparent photoconductive film or layer 14 may be composed of any suitable material such as, for example, polystyrene resin containing a dispersion of a photoconductive material such as fine particles of copper doped cadmium sulde or cadmium selenide, for example. Since the recording operation of this invention does not depend upon thermal development of a ripple or groove pattern in the surface of the photoconductive film, thermosetting resins such as transparent phenolic resins or transparent epoxy resins may be used in place of the polystyrene. The photoconductive layer may be formed by dissolving the desired amount of the resin in an appropriate volatile solvent such as, for example, benzene or methyl ethyl ketone along with a curing agent where required and adding the finely divided photoconductive material thereto, stirring to form .a suspension. The liquid may then be cast upon the electrically conductive film to form a liquid layer which is then heated to evaporate the solvent. Satisfactory films prepared in this manner may be as thin as 1 to 2 microns.

With reference to FIGURE 1, the invention may best the illustrate by considering the figure from left to right. As schematically shown, electrically conductive layer 13 is vconnected to ground and a uniform electrostatic charge is applied to the photoconductive layer 14, as indicated by the plus (-1-) signs, by any effective means, such as a corona discharge, for example. An image pattern of colored light is then caused to impinge upon the uncoated surface of the transparent substrate 11 as indicated by the arrows 15. The relative length of the arrows 15 is intended to represent relative light intensity, e.g., the higher the intensity, the longer the corresponding arrow. The incident light passes through the substrate 11 and encounters the screen plate 12 whereupon the filters act to block or to pass the light striking them depending upon the color characteristics of the light incident upon each filter element. For example, monochromatic blue light will be passed by blue filter elements but blocked by the red and green elements, red will be passed by the red elements but blocked by the blue and green elements, and of course, green will pass the green elements but not the red and blue elements. In the case of polychromatic light such as white light, for example, the red, green, and blue components will be passed by the corresponding filter elements. Light which is passed by the screen plate 12 strikes the photoconductive layer 14 and reduces the electrical conductivity of the area of the layer so illuminated allowing the electrostatic charge associated with that area to be dissipated. The degree of charge dissipation occurring at any given area will be proportional to the intensity of the light to which the area is exposed. This has been schematically illustrated in the second stageY of FIGURE l where, for example, in those areas associated with the monochromatic blue light, no charge is lost in those areas which are associated with red and green filter elements while the amount of charge lost in the areas associated with the blue filter elements is proportional to the intensity of the blue light incident thereupon. Corresponding behavior for red and green light is also shown. In the area which was not illuminated, no charge is lost as shown. The behavior of the screen plate with respect to white light of different intensities is also shown.

After exposure to the light pattern or image of colored light, negatively charged electroscopic powder shown schematically at 16 as a plurality of negative signs is dusted upon the surface of layer 14 bearing the'rernanent charge pattern. While a number of electroscopic powders may be employed, a mixture of Toner Type 10 and Developer Type 10, both marketed by the Xerox Corporation, Rochester, N Y., has been found to be satisfactory for the practice of the invention. The powder is electrostatically attracted to the charged areas and adheres thereto in amounts proportional to the strength of the charge on any particular area. As previously disclosed, preferably the electroscopic powder contains an adhesive material which may be softened at a relatively low temperature such as for example about 50 C. to 100 C. and upon cooling forms a coating which is composed of the individual particles adhesively secured to each other and to the surface of the photoconductive layer 14 in the form shown at 17. The thickness of the coating 17 varies from area to area in proportion to the strength of the remanent charge pattern associated with any particular area, and since the strength of the charge pattern associated with any particular area was proportional to the intensity of the light transmitted to it by its associated filter element or elements, the thickness of any particular area of coating is related to the color of the irradiating light impinged upon it but to the intensity of the light as well. As previously stated, the electroscopic powder used is opaque and forms a coating -17 which varies in its ability to transmit light in an inverse proportion to its thickness. It will, of course, be appreciated that the relative thickness of coating 17 as shown has been greatly exaggerated for purposes of illustration. Thus, when the uncoated surface of substrate 11 is illuminated uniformly with white light as shown in the right hand portion of FIGURE l, light is transmitted through the recording member and coating 17 in an image pattern of colored light which corresponds very closely with the image pattern of color which was originally projected upon the recording medium, both as to form and intensity of color, which image pattern may be projected for viewing using a conventional slide projector or the equivalent.

It will be appreciated that the screen plate filter elements are Very small in area or in transverse dimension and may be made in a variety of forms. For example, the screen plate illustrated in FIGURE 2 may be made from dyed starch grains about 0.015 mm. in diameter. Equal volumes of red, green, and blue grains are mixed together to form a uniformly gray powder which is dusted over a transparent substrate surface made tacky with an adhesive such as a clear varnish. The excess powder is removed and the remaining adherent starch grains flattened by a pressing operation to form a filter screen plate as shown. The interstices may then be filled with a strongly absorbent material such as a fine carbon powder or graphite. If desired, colloidal resin particles `may be substituted for the starch particles. Other ways of forming equivalent screen plates may be employed and are more fully disclosed in the concurrently filed application Ser. No. 472,581 previously referenced. For example, instead of forming the screen plate from a more or less statistically random distribution of the irregularly shaped primary color filter elements are previously disclosed, the filter elements may have a regular geometric shape such as small squares or circular dots, for example, and may be arranged in a regular pattern. Furthermore, acceptable screens may be formed from parallel line elements of the primary colors. These variations may be made by a variety of methods and techniques, all disclosed in greater detail in the aforementioned concurrently filed application. In any event, the maximum transverse dimension of each filter element i.e., the width of line elements or the mean diameter of dot-like elements, should be less than about 0.005 inch.

In another embodiment of the invention, a photoionizable material such as iodoform, for example, is substituted in the photosensitive layer 14 for the photo-conductive material. In this modification, the so-called photocharge phenomenon is utilized instead of the photoconduction phenomenon previously described. In this case, an analogous recording medium as shown in FIGURE 3, is composed of a transparent substrate 11 which is identical to that disclosed with respect to FIGURE 1 is similarly provided with the same sort of screen plate 12', a similar conductive film 13 but with a photosensitive layer 14' composed of a photocharge material in a plastic matrix. A complete description of such a recording medium for black and white powder development of light images is set forth in copending U.S. application Ser. No. 311,160, filed Sept. 24, 1963 and now abandoned in the names of Joseph Gaynor and Gordon I. Sewell, entitled Information Recording, and assigned to the assignee of the present invention, except that the concept of utilizing a screen plate for producing color transparencies is not disclosed. In this embodiment of the invention, when recording medium 10 is exposed to a colored light image pattern as previously described with respect to the ernbodiment shown in FIGURE 1, a latent charge pattern is formed within the photosensitive layer 14 which corresponds to the light pattern passed by the screen plate 12 and is schematically shown by shaded areas 20. As such, there are no surface charges capable of attracting and holding electroscopic powder at this time. If an electrode member 21 which has a conductive area coextensive with the photosensitive layer 14' but spaced a short distance therefrom is connected to one pole of a source of direct current such as a battery 22 and the other pole of the battery is connected to the conductive layer 13 as shown, a field is created across the air gap between layer 14 and electrode 21. The field so produced is modulated by the latent charge pattern 20` and if electroscopic powder of the appropriate polarity is dusted into the air gap while the field is applied, it will be attracted to the surface of the photosensitive layer 14 and adhere thereto in a pattern completely analogous to that shown in FIGURE 1, where it may be secured in place to form a similar coating by heating for example. If an electroscopic powder is used having a negative potential, a positive potential is required to be induced at the photosensitive surface. Reversing the potential of the powder requires a reversal of the sign of the induced potential on the surface.

In a still further variation of the invention, the exposed photocharge layer 14 may be electrostatically charged by means of a corona discharge and the latent charge pattern 20 will modulate the remanent electrostatic charge pattern in the same manner as shown in the second part of FIGURE 1. Again, a coating having a varying thickness and therefore the ability to transmit light which is modulated according to the original color light image may be formed as previously described.

All the foregoing examples of the invention have involved the use of a laminated structure in which the screen plate has -formed an integral, permanently affixed part of the recording member. It should be appreciated that byl so doing, registration problems are eliminated, however, it should also be appreciated that the recording layer may be made as a separable element so that the substrate and screen plate may be separated and not subjected to heating during the formation `of the coating by the heating of the powder. In this case, after the coating is formed, the substrate and the screen plate may be reassembled with the recording layer, however, proper registration may require some trial and error adjustments before a perfect image is achieved.

A yet further embodiment of this invention entails the use of what may be termed an optical screen plate. This is illustrated in FIGURE 4 in which a banded filter 30 composed of a red band 31, a green band 32, and a blue band 33 is positioned as shown with respect to any suitable optical lens system 35. The lens system focuses polychromatic light which has been broken up into the three primary colors by the banded filter 30 onto the lens-like surfaces 36 of a recording member 37. Recording member 37 is composed of a lenticular supporting screen member 38 composed of a transparent material such as, for example, Mylan which is provided on its free surface with cylindrical lens-like deformations 36 as shown. These deformations extend across the width of the support member and may be of the order of 600 per linear inch and extend out of a plane determined by the bottoms of the trough-like intersections of the curved surfaces of the lens-like deformations. Such transparent lenticular film is marketed under the trade name of Kinescope by the Eastman Kodak Company. On the opposite surface of member 38 is a photosensitive layer 39 which may be similar to layer 14 of FIGURE l or layer 14 of FIGURE 3. When an image pattern of polychromatic light passes through the banded filter 30, the image pattern is broken up into the three primary colors and is focused by the lens system 35 upon the individual lens-like embossings 36 which are contoured to focus the light received theron as very small images of the banded filter in the recording layer 39 under each of the embossings, as shown at 40. A transparent electrically conductive film 41 is also provided as shown. The light incident upon each area under each lens-like embossing 36 can react with the photosensitive layer in the same manner as described in connection with FIG- URES 1 and 3 and a modulated coating layer formed from adhered electroscopic powders as aforesaid. The image pattern so recorded may then be reproduced in faithful color and form by projecting white light through a banded filter such as filter 30, an appropriate lens system, through the developed recording member 37 and through a conventional projector.

From all the foregoing, it will be apparent to those skilled in the art that the present invention provides a solution to the pre-existing problem of providing color images having a point-to-point correspondence with an image pattern which was recorded by xerographic methods with regard to color, form, and intensity of color. It will, of course, be appreciated that if the supporting layer or the recording layer of the recording medium are composed of materials which have a slight absorption in the visible range of the spectrum, then an appropriate color balance may be achieved by suitable alternations vof the color composition of the screen plate. Further, while the electrically conductive layer 13 has been described and shown as located .between the recording layer 14 and the screen plate 12 in the embodiment of FIG- URE 1, if the screen plate is considerably more conductive than the photoconductor in the dark, then the physical arrangement of the conductive layer and the screen plate may be reversed, i.e., by locating the conductive layer 13 between the substrate 11 and the screen plate 12. In view of the many variations of the invention which will occur to those skilled in the art, it is not intended that the scope of the invention be limited to any one or all of the specific embodiments which have been disclosed for exemplary purposes but only by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method for recording an image pattern of colored light comprising:

(a) projecting an image pattern of colored light through a filter composed of areas of three primary colors to produce an image pattern composed of small areas of primary colored light;

(b) causing the image pattern of primary colored light areas to impinge upon a film of a transparent, electrically photosensitive material;

(c) producing a modulated pattern of areas capable of attracting and retaining electroscopic powder upon an exposed surface of said photosensitive film, said attractive areas having a point-to-point correspondence with said colored light areas with respect to configuration and having relative levels of attraction proportional to the intensities of the related light areas;

(d) dusting over said surface an electroscopic powder having a charge which attracts said powder to said attractive areas;

(e) removing excess electroscopic powder leaving an attached powder layer of modulated opacity on said surface, said powder layer having greatest opacity over those areas which were more strongly attractive; and

(f) fixing said powder to said surface as a coherent layer.

2. The method recited in claim 1 in which said filter is composed of a plurality of areas which respectively and selectively transmit red, green, and blue colored light.

3. The method recited in claim 2 in which said filter is composed of a substantially statistically random distribution of said areas.

4. The method recited in claim 2 in which each of said areas has a transverse dimension less than about 0.005 inch.

5. The method recited in claim 4 in which said image pattern of colored light is sequentially projected through a filter consisting of three approximately equal areas which respectively and selectively transmit red, green, and blue light and through a transparent member having a surface upon which said filtered image impinges consisting of a plurality of minute lenses to produce said image pattern composed of small areas of primary colored light.

6. The method recited in claim 1 in which said pattern of attractive areas are produced by electrostatically charging said exposed surface.

7. The method recited in claim 1 in which said pattern of attractive areas are produced on said exposed surface by applying an electrical field across said surface.

8. The method recited in claim 1 in which said powder is fixed to said surface by heating said powder and surface to cause a constituent thereof to soften to form an adhesive bond, followed by cooling.

9. A method for reproducing the image pattern of colored light recorded in claim 1 comprising projecting a uniform beam of white light through said filter and said coherent layer of powder having modulated opacity to produce a projected image pattern corresponding to said originally projected image of colored light.

10. A color recording medium comprising in combination a transparent support member having a pair of substantially parallel planar major surfaces, a transparent photosensitive film supported by one of said surfaces composed of a material capable of exhibiting large changes in electrical properties in areas thereof which are exposed to light in a substantially panchromatic manner, a transparent electrically conductive film interposed between said support member and said film, and means to cause an image pattern of colored light to be impinged upon the surface of said film adjacent to said support member in the form of minute areas of primary color for each elemental area of the image pattern of colored light.

11. A recording medium as recited in claim 10 in which said means includes a transparent filter layer positioned adjacent one of said major surfaces of said support member, said filter layer being composed of a plurality of areas which respectively and selectively transmit red, green, and blue colored light.

12. A recording medium as recited in claim 11 in which said filter layer is positioned between said support member and said film.

13. A recording medium as recited in claim 12 in which each of said areas of said filter layer has a transverse dimension less than about 0.005 inch.

14. A recording medium as recited in claim 13 in which said filter layer is composed of a substantially statistically random ydistribution of said areas.

15. A recording medium as recited in claim 11 in which said means includes a transparent filter consisting of three approximately equal areas which are adapted to respectively and selectively transmit red, green, and blue light therethrough, means for focusing such transmitted light upon the other of said pair of major surfaces of said support member and said other'surface being pro- 9 vided with a plurality of minute lenses which are adapted to produce an image pattern on said film in the form of small areas of primary colored light.

References Cited UNITED STATES PATENTS 1,965,852 7/1934 Miller 96-80 3,113,022 12/1963 Cassiers et al. 96--1 FOREIGN PATENTS 372,247 1962 Japan.

10 OTHER REFERENCES OTS: U.S. Dept. of Commerce Translation Scientic Lib., 10-101963. All pages. Translation of Japan patent publication 2247-62.

GEORGE F. LESMES, Primary Examiner JOHN C. COOPER III, Assistant Examiner 

